Late pliocene dawson cut forest bed and new tephrochronological findings in the Gold Hill Loess, east-central Alaska

Thick loess deposits in the Fairbanks region of interior Alaska are a rich source of information on past climates and environments during the late Cenozoic, and the numerous distal, silicic tephra beds preserved within them offer the potential for good chronological control. The Dawson Cut Forest Bed lies in the lower part of this loess cover. Plant macrofossils consist of Picea glauca, Picea mariana, Betula papyrifera, and Populus balsamifera but no Pinus. These fossils, together with the abundance and size of spruce remains, pollen, tree-ring characteristics, and δ¹³C values of spruce wood, demonstrate that the boreal forest represented by the Dawson Cut Forest Bed was similar to the modern boreal forest of central Alaska. Warming conditions during the early part of the Dawson Cut Interglaciation initiated thawing of permafrost and melting of ground ice, as evidenced in the presence of ice-wedge casts and major erosion of the lower Gold Hill Loess. Tephrochronological, magnetostratigraphic, and glass fission-track dating studies in the Fairbanks area and at the Palisades site on the Yukon River in central Alaska suggest an age for the Dawson Cut Forest Bed of ca. 2 Ma. Hence, the northern boreal forest of northwestern North America, as we know it today, has a long history that probably extends back to at least 2 Ma.